This invention relates to an electrostatic copying apparatus and more particularly to an electrostatic copying apparatus which is provided with an electrifying device capable of defining a surface potential of a photosensitive layer during electrification.
An electrostatic copying apparatus is generally provided with a photosensitive drum, which comprises a drum body acting as a conductive support and a photosensitive layer which is prepared from a photoconductive material and mounted over the peripheral surface of the drum body. The photosensitive layer undergoes the sequential copying steps of electrification, exposure to light, development, transcription, cleaning and discharge. The electrification step is carried out by the electrifying device.
Electrification by the electrifying device is generally carried out by exposing a photosensitive layer to a corona discharge, and depositing ions emitted from the corona discharge on the surface of the photosensitive layer. In the corona discharge a high voltage of several thousand volts is impressed on a space defined between a fine conductor set immediately above the photosensitive layer and the drum body. A corona emitted from the conductor ionizes the surrounding air molecules to generate ions. Deposition of ions on the photosensitive layer gives rise to its surface potential. The surface potential prominently affecting the concentration of a transcribed image should always be stabilized.
In this connection, it will be noted that the photosensitive layers of successively replaced photosensitive drums, though prepared from the same material, indicate some variations in the static electrical characteristic. Even when an exactly fixed voltage is impressed, the surface potentials of the photosensitive layers have hitherto varied within the range of .+-.50 V for each photosensitive drum. Where, therefore, an old photosensitive drum whose photosensitive layer is already deteriorated is replaced by a new photosensitive drum, then the photosensitive layer of the new photosensitive drum may have a higher or lower surface potential than that of the photosensitive layer of the old photosensitive drum which originally proved satisfactory. In such case, the concentration of an image transcribed by the new photosensitive drum will change, resulting in a decline in the picture quality of the image.
For stabilization of the surface potential of the photosensitive layer, therefore, it has hitherto been necessary to adjust an output from a voltage-impressing transformer or a distance between a corona-producing conductor and photosensitive layer. In either case, adjustment has been carried out very unsatisfactorily. In the former case, the surface potential widely varies with any slight change in an amount of electric energy discharged by voltage impression. For the stabilization of the surface potential, therefore, an output from the voltage-impressing transformer has to be very minutely adjusted. Where, in the latter case, a distance between the conductor and photosensitive layer varies by about one millimeter, then the surface potential indicates as wide a change as about 100 V. Therefore, the adjustment of the distance demands an advanced skill. Further, if the adjustment proves unsatisfactory, then irregular electrification undesirably results.